A Dietitian Answers: Is Intermittent Fasting a Boost or a Bonk for Runners?

Intermittent fasting has gained popularity recently, and while it is touted to promote weight loss, improve insulin sensitivity, and have other metabolic benefits, is it right for runners?

What is intermittent fasting?
While there are lot of versions of intermittent fasting, two strategies take front stage. The 5:2 fasting method allows for “normal” eating five days a week and fasting days two days a week. In this scenario, fasting means consuming 500–600 calories per day.

The other popular version of intermittent fasting is limiting eating to 6–8 hours per day. This means that you consume all your daily calories between 12–6 p.m. or 10 a.m.–6 p.m. Basically, it limits your eating window and encourages quality food choices, which in turn decreases total calorie intake over the course of the day.

Can it help runners?
If you are looking to lose weight and thus run faster, intermittent fasting might help. However, most of the research conducted is in the overweight and obese population with less research done in active, fit individuals. Because of this, it’s hard to determine if intermittent fasting can actually boost a runner’s performance. Considering food is a runner’s energy source, it seems less likely.

Can it cause bonking for runners?
Fasting can compromise energy levels and recovery time in runners, and long term it could contribute to decreased performance. Carbohydrate helps fuel your runs and replenish energy stores, while protein helps repair damage to lean muscle mass. Without these essential ingredients, it is likely you may bonk in runs and not recover as quickly.
Fasting might also lead to binging during eating times and is likely not sustainable for the long haul. If weight loss is a goal, it is better for runners to create a small deficit each day versus fasting. This can help you meet your weight goals while not compromising your runs.